Kinetics in the real world
Reaction rates determine how long food lasts, how drugs work, how pollution forms, and how industry makes chemicals efficiently.
Enzymes and life
Nearly every reaction in the body is catalysed by an enzyme. Enzymes bind reactants (substrates) at an active site shaped to stabilise the transition state. Carbonic anhydrase, which helps remove carbon dioxide from blood, processes hundreds of thousands of molecules per second.
Enzyme kinetics
At low substrate concentration, enzyme reaction rate rises with substrate; at high concentration, all enzymes are busy and the rate levels off at a maximum, \(V_{\max}\). The Michaelis constant \(K_M\) is the substrate concentration giving half the maximum rate. Many drugs work by inhibiting specific enzymes.
Drug elimination
Most drugs are eliminated by first-order kinetics, with a constant half-life. Dosing schedules are designed to keep drug levels within a therapeutic window: high enough to work, low enough to avoid toxicity. After about five half-lives, roughly 97% of a drug is gone.
Atmospheric chemistry
The ozone layer was depleted by chlorine from chlorofluorocarbons (CFCs), which acts as a catalyst: one chlorine atom can destroy many thousands of ozone molecules before being removed. The Montreal Protocol phased out CFCs, and the ozone layer is slowly recovering.
Industrial catalysis
The Haber process makes ammonia from nitrogen and hydrogen using an iron catalyst at high temperature and pressure. Catalysts are central to petroleum refining, plastics manufacture, and pollution control, and most industrial chemical processes rely on them.
Food chemistry
Browning of cut fruit is an enzyme-catalysed oxidation; lemon juice slows it by lowering pH and blocking the enzyme. The Maillard reaction between sugars and amino acids gives roasted coffee, toasted bread, and seared meat their flavours; its rate rises sharply with temperature.
Explosions and controlled reactions
Explosions are extremely fast exothermic reactions, often producing large volumes of gas. Engineers control reaction rates in engines and rockets by managing mixing, temperature, and surface area, turning the same chemistry into useful, steady power.
Worked example
A drug has a half-life of 4 hours. After 12 hours (3 half-lives), \(\left(\frac{1}{2}\right)^3 = 12.5\%\) remains. After 20 hours (5 half-lives), about 3% remains, so the drug is considered essentially eliminated after roughly a day.
Key takeaways
Enzymes catalyse life's reactions and saturate at \(V_{\max}\); inhibitors are key drugs. Drugs usually follow first-order elimination with constant half-lives. Catalysts underpin industry and pollution control, and catalytic chlorine destroyed ozone. Temperature strongly affects food chemistry and safety.
Flashcard
Region of an enzyme that binds substrate
Check your understanding
Why does lemon juice slow the browning of cut apples?